Rinse each fruit and split in half with a knife, without peeling. Trim rind slightly so that each half lies flat. Using a sharp paring knife, separate the flesh from the membrane of each fruit. Spread ½ tbsp of butter on each fruit half, and sprinkle with ground cinnamon. Place fruit in a cookie sheet or broiler pan, and put under the broiler. Cook until heated through and top begins to caramelize.